1. Field of the Invention
This invention relates to fluid propellant propulsion systems and methods. Particularly, this invention relates to such propulsion systems and methods in space applications.
2. Description of the Related Art
A variety of liquid propellant systems have been proposed and developed to drive rockets and space vehicles. In most liquid propellant rocket engines, a fuel and an oxidizer, e.g. kerosene and liquid oxygen (LO2) are pumped into a combustion chamber where they burn to yield a high pressure and high velocity gas stream. The flow of the gas through a nozzle accelerates it further until it exits the engine. The exiting gas provides thrust in the opposite direction which is used to accelerate or maneuver the vehicle.
In many space vehicles it is typical for the fuel and/or the oxidizer to be a cryogenic liquefied gas such as liquid hydrogen or LO2. A common problem in a liquid propellant rocket engine is cooling the combustion chamber and nozzle. Accordingly, the cryogenic liquids are often circulated around the super-heated parts in order to cool them. The pumps must generate extremely high pressures to overcome the pressure that the burning fuel creates in the combustion chamber.
Many different combinations of fuel and oxidizer have been used in liquid propellant rocket engines. For example, gasoline and liquid oxygen were used in early rockets of Goddard. Kerosene and LO2 were used in the first stage of the large Saturn V boosters in the Apollo program. Liquid hydrogen and LO2 are currently used in the Space Shuttle main engines. And nitrogen tetroxide and monomethyl hydrazine were used in the Cassini mission to Saturn.
Recently, there has been interest in propulsion employing a combination of LO2 and liquid methane (LCH4). The bipropellant of LO2 and LCH4 has recently been selected by NASA as a possible fuel for the Crew Exploration Vehicle (CEV) and future space exploration. A fundamental physical problem in developing a propulsion system employing this bipropellant is storing the cryogenic LO2 and liquid methane (LCH4) with the least amount of boil-off due to heating and with the least amount of mass and power required. Another problem is providing a means to drain single-phase liquid from the storage tank without an entrained gas phase. In addition, the system must maintain the storage tank within a specified pressure and temperature range while the gravitational environment varies from zero-gravity to accelerations much larger than Earth's normal gravity.
Because both fluids are cryogenic, typical thermal environments on Earth and in space will cause the propellants to warm and tend to boil within the tanks. As the pressure nears the structural limits of the tank, it must be reduced, either by venting or some other means. Limiting the amount of heat flow into the tanks prolongs the lifetime of the cryogenic liquid because boil-off and the associated pressure increase is directly related to the amount of energy flow into the storage vessel. An active refrigeration system or cryocooler can be employed to intercept the external heat flow and maintain the tanks at sufficiently cold temperatures. However, such cryocoolers require relatively high electric power and generally operate continuously. For spacecraft and other energy limited applications, large power consuming systems are undesirable.
Other more passive techniques that condition the fluids without the energy consumption of a cryocooler are known, but they typically operate with less cooling performance. However, for applications without long lifetimes, a passive thermal solution may be a better solution. In such passive systems, foam and multilayer insulations have been used as well as low-conductivity structural supports and vapor-cooled shields. For applications where tank mass is less critical, a dual wall container can be used with an evacuated cavity to minimize wall heat flow.
Another challenge in developing an oxygen and methane bipropellant system involves the draining of liquid tanks in low-gravity or highly dynamic acceleration environments to acquire a single phase liquid. Draining liquid from a tank on Earth or in steady elevated acceleration fields is performed by simply placing an outlet at the bottom of the tank. However, in low gravity, with no significant gravity field to pull it to one side of the tank, the specific liquid location within the tank is generally not known at all times because the liquid can easily move about the tank. To deal with this problem, special liquid acquisition devices (LADs), which operate based on the surface tension properties of the fluid, are often employed to address the low-gravity liquid dynamics.
For example, U.S. Pat. No. 5,901,557, issued May 11, 1999 to Grayson, which is incorporated by reference herein, discloses a vessel storing cryogenic fluid having a passive thermodynamic venting system for effectively and reliably transferring heat in a reduced-gravity environment. The storage vessel has a storage tank for holding the cryogenic fluid under pressure. The storage vessel is compartmentalized using a screen trap so that the heat exchanger of the venting system extends through a compartment which includes only the liquid phase of the cryogenic fluid. A screen gallery, screen trap and vane assembly cooperate to separate the gas and the liquid phases of the cryogenic fluid. The thermodynamic venting system includes a throttle device for reducing the temperature of cryogenic fluid. A conduit in contact with heat exchange elements transfers heat from the liquid phase of the cryogenic fluid to a relief valve for venting the heat external of the storage tank.
Grayson, “Propellant Trade Study for a Crew Space Vehicle”, AIAA 2005-4313,41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit 10-13 Jul. 2005, Tucson, Ariz., which is incorporated by reference herein, discloses a trade study to determine the best propellant combination for a notional crew space vehicle. The assumed 5000 ft/s spacecraft is divided into a command module and service module like Apollo and provides transportation of astronauts and supplies to low Earth orbit, the International Space Station, libration point one, and one-way transfer from lunar orbit to Earth. Twenty-five different propellant combinations are evaluated across nine important evaluation criteria that include mass, development, safety, complexity, reliability, flexibility, contamination, commonality, and Mars in-situ producibility. Nontoxic and Mars-producible are decided to be important requirements for an affordable Earth-moon-Mars exploration architecture. The assumptions when coupled with a mathematical model to estimate vehicle wet mass, lead to the recommendation of liquid oxygen and liquid methane for orbital maneuvering and gaseous oxygen with gaseous methane for reaction control. The new propellant combinations require up-front investment that includes new or modified engines, ground infrastructure, long term cryogenic storage technology, and, for the later occupation of Mars, in-situ production of methane and oxygen for propulsion.
In a conventional storage system applied to LO2 and LCH4 bipropellant, the liquids are stored in separate tanks with separate thermal conditioning hardware. In this case, each tank requires separate insulation, thermodynamic vent, vapor-cooled shields, and separate cryocoolers (for long duration storage). In such separate, thermally independent tanks, each fluid is typically stored at its normal boiling point in one atmosphere of pressure which is about 162° R for LO2 and 201° R for LCH4. Thus, a conventional solution requires more insulation due to larger tank external surface area and additional thermal conditioning hardware. This results in a higher total mass of the tanks and the associated thermal conditioning hardware.
In view of the foregoing, there is a need in the art for systems and methods for cryogenic storage of liquid propulsion constituents which require less mass. There is also a need for such systems and methods to operate more efficiently, operating with significantly lower power requirements. Particularly, there is a need for such systems and methods for LO2 and LCH4 bipropellant systems. As detailed hereafter, these and other needs are satisfied by embodiments of the present invention.